1. Field of the Invention
The present invention relates to the simultaneous casting of multiple alloys, in particular, direct chill casting of multiple aluminum alloys using a metallic member between the alloys to form a multi-component cast product and/or the use of a metallic member as an external layer on a cast ingot.
2. Prior Art
In the production of aluminum alloy ingots by a conventional direct chill (DC) casting process, molten aluminum is poured into an opened end mold. The lower end of the mold is initially closed by a platform referred to as bottom block and the molten metal pools within the mold. The bottom block is progressively lowered in step with the pouring of the molten metal. The wall of the DC mold is continuously cooled so that a solid skin of metal forms in contact with the mold wall at the level of the surface of the pool of molten metal in the mold. An example of the method of DC casting is described in U.S. Pat. No. 4,071,072, incorporated herein by reference. In this conventional operation, a single molten aluminum alloy is direct cast into an ingot.
Such aluminum ingots are often times incorporated with other alloys to form a composite product. For example, brazing sheet for the header of a heat exchanger or for reinforcement structures may be produced from an Aluminum Association (AA) 3000 series aluminum alloy with a clad layer of an AA 4000 series alloy. Evaporator sheet product or plate type heat exchangers typically include a 3000 series alloy clad on both sides with a 4000 series alloy. Likewise, radiators often are formed from a 3000 series alloy with a 4000 series cladding and water-side liner of an AA 1000, 5000, 6000, or 7000 series alloy. The clad layer is conventionally roll bonded in plate form onto an ingot of the core alloy (e.g., a 3000 series alloy). Roll bonding requires multiple rolling passes, scalping, reheating, and sealing steps to produce the clad alloy in sheet form. Each of those processes adds to the cost of the final clad product. In addition, the thickness of cladding produced via roll bonding is generally limited to a maximum of only about 35% of the total sheet thickness. Roll bonding can also be extremely difficult if the mechanical properties of the alloys being roll bonded are too dissimilar at the rolling temperatures. For example, when one alloy deforms very easily while the other alloy does not, the alloys do not seal properly or the target cladding ratio is off.
More recently, attempts have been made at casting composite metal products. One such process is described in DE 4420697 in which one alloy of a billet is DC cast on one side of a fixed barrier and another alloy is DC cast on the opposite side of the barrier. The process is controlled such that the two molten metals come in contact with one another while in the molten state to provide a controlled mixing of the two melts. In this manner, the composition of the composite billet in the direction perpendicular to the contact surface of the two metal components changes continuously. The concentration of the individual alloy elements changes continuously from the values of one alloy to the values in the other. The fixed barrier maintains the two components apart from each other within the mold, and the barrier is positioned off center so that one component is narrower than the other. The alloy closest to the mold (the narrower component) cools and solidifies earlier in the process than the other alloy, i.e., at a great height from the bottom block. The bottom block is withdrawn at a speed whereby the levels of the melts within the mold remain approximately even. Although one alloy solidifies before the other alloy, there is a small region between the melts in which the melts are able to flow into one another and mix briefly to promote adhesion between the two alloys. While this method provides some adhesion between the two components of the cast product, the mixing of the components which occurs during the casting can be detrimental to the finished product. The location and shape of the fixed barrier are also critical to avoid intermixing of the molten alloys. The properties of the alloys simultaneously cast in this manner may be affected by the mixing of the alloying components. This method also requires careful control of molten metal flow to avoid mixing due to hydraulic pressure differences as well as careful control of the solidification rate of the alloy forming the narrower component to ensure only brief mixing of the alloys in the region immediately below the barrier.
Another method of DC casting a composite ingot is disclosed in U.S. Pat. No. 4,567,936 in which an outer layer is simultaneously cast within an inner component. According to this method, the outer layer solidifies prior to contact within the molten inner alloy. This avoids mixing between the components of the inner component and the outer layer. A drawback to this method is that the outer layer must solidify completely before the inner alloy can be cast within the outer layer. The thickness of the outer layer also is limited because the heat of the inner component must exit through the outer layer to the exterior surfaces of the cast product. Hence, the configuration of the final multi-component product also is limited.
Accordingly, a need remains for a method of simultaneously casting a multi-alloy metal product with a minimum of mixing between the alloys of the product and which can produce cast metal products in a variety of configurations.